The present invention relates to a surface shape evaluation method and device for a spherical body, which is used for judgement, good or not, of the surface shape of a ball for incorporation in a ball bearing for use in a rotation support portion of e.g. a hard disc drive device (HDD) or video tape recorder (VTR).
Used in the hard drive device (HDD) or video tape recorder (VTR) is a ball bearing 1 as shown in FIG. 9, which comprises an outer ring 2 having an inner peripheral surface formed with an outer ring raceway 3, an inner ring 4 having an outer peripheral surface formed with an inner ring raceway 5, a plurality of balls 6 rotatably provided between the outer and inner ring raceways 3, 5 and held in a cage 7, so that the outer ring 2 and inner ring 4 are rotated relative to each other.
The ball bearing 1 for use in the hard disc drive device (HDD) or video tape recorder (VTR) must be provided with extremely high rotation precision. For example, in the case of the ball bearing 1 to be incorporated in the motor
However, there are minute lands and recesses (undulation) on the rolling surface of the balls 6 for use in the ball bearing 1, which could not be avoided in processing. And, the distance between the outer ring raceway 3 and the inner ring raceway 5 corresponding to the diameter of the balls 6 changes slightly during a single rotation of the balls due to undulation. As a result, the positional relationship of the inner ring 3 with respect to the outer ring 1, and vice versa, changes slightly as shown by the curved lines in FIG. 10. This is referred to as runout. Incidentally, the displacement appearing in the ordinate in FIG. 10 is the vibration amplitude of the ball bearing 1. This runout is repeated every rotation of the inner ring 4 (or of the outer ring 2). In addition, the amount (displacement amount) and location of runout slightly changes, which is so-called non-repetitive run out (NRRO). In other words, the relation in location between the outer ring 2 and the inner ring 4 is not reproduced and vibrations caused change from rotation to rotation. Consequently, as the integrity of information on the magnetic disc becomes high, errors can be produced in reading and weighting by the magnetic head. Therefore, the performance of the ball bearing 1 is a bar against the high density of the HDD.
Accordingly, the non-repetitive runout of the ball bearing 1 must be minimized to obtain a high density HDD. The roundness of the balls 6 must be improved to make the waviness of the rolling surface of the balls 6 small so as to minimize the non-repetitive runout. On the other hand, it is substantially impossible on technology to further improve the process precision of the balls 6, and therefore to minimize the non-repetitive runout by improving the roundness of the balls 6. Even if it is possible to further improve the roundness, it will be costly, resulting in high production cost of the ball bearing 1 having the balls 6 incorporated therein, and of the HDD having the ball bearing 1 incorporated therein.
Under such a situation, a ball bearing with small non-repetitive runout is disclosed in JP Patent Publication Toku Kai Hei 8-24715 where the roundness of the balls 6 is not particularly improved. The ball bearing disclosed in the publication is provided with one of the following conditions:
1. With the components of the number of waves per circumference on the rolling surface of the ball 6, the sum of the even orders of waviness is smaller than the sum of the odd number of waviness.
2. With the components of the number of waves per circumference on the rolling surface of the ball 6, the maximum in half amplitude of the even orders of waviness is smaller than the maximum in half amplitude of the odd orders of waviness.
3. With the components of the number of waves per circumference on the rolling surface of the ball 6, the sum of the even orders of waviness is smaller than the sum of the odd orders of waviness, and in addition the maximum in half amplitude of the even orders of waviness is smaller than the maximum in half amplitude of the odd orders of waviness.
With the rolling surface of the balls 6 in the ball bearing disclosed in Toku Kai Hei 8-247151, the non-repetitive runout can be made small because the sum of the even orders of waviness and the maximum in half amplitude of the even orders of waviness having larger bad effects on the vibration characteristics are smaller than the sum of the odd orders of waviness and the maximum in half amplitude of the odd orders of waviness, respectively. The sum of the odd orders of waviness and the maximum in half amplitude of the odd orders of waviness are not necessarily made smaller than in the prior art because the do not badly affect the vibration characteristics. Therefore, even if the roundness of the balls is not particularly improved, the vibration characteristics are improved and non-repetitive runout can be made small. Toku Kai Hei 8-247151 is incorporated in this specification by reference, which describes the reasons of the improvement in vibration characteristics achieved, by making small the sum of the even orders of waviness and the maximum in half amplitude of the even orders of waviness even if the sum of the odd orders of waviness and the maximum in half amplitude of the odd orders of waviness are not necessarily made small.
The invention as shown in Toku Kai Hei 8-247151 can realize a ball bearing and HDD, etc., with higher performance and lower cost. However, it is necessary to efficiently judge the shape of the rolling surface of the balls 6 produced whether it conforms to the conditions described in the publication.
Conventionally, a general shape measurement device is used for judgement, good or not, of the rolling surface of the balls 6, where the error in rotation of the spindle to rotate the balls 6 to be detected is directly associated with the measurement error. In addition, in the worst case, the rotation error may be larger than the components of waviness. Accordingly, the rotation precision of the spindle must be sufficiently made larger, so that the production cost of the measurement device is unavoidably increased. On the other hand, with the waviness measurement devices available in the market, it is difficult to secure the rotation precision, and specifically to precisely evaluate the low frequency components of the surface shape, and in addition wear the bearing portions for the ball is severe, so that rotation of the ball is unstable, and precise measurement is difficult, and reproductivity is difficult in measurement.
In the prior art devices, the waviness of the rolling surface of the balls is evaluated by summing the components of waviness which have passed stationary filters having appropriate band-pass frequency ranges. Accordingly, it is impossible to evaluate specific components of waviness badly affecting the vibration characteristics of the ball bearing. For example, ball bearings having larger odd orders of waviness are rejected as poor even when its even orders of waviness are small. Therefore, it is impossible to evaluate the balls 6 for the ball bearing which must be optimum for the characteristics of the instruments such as HDDs where the ball bearing is incorporated.
In addition, the conventional measurement devices have a probe the tip end of which is sharp or of a spherical convex shape, so that it is difficult to locate the tip end of the probe in alignment with the portion to be detected for evaluation of tiny balls 6. Specifically, in measurement of the waviness of the rolling surface of the ball 6, the tip end of the probe is abutted to a so-called equator portion of the ball 6 during rotation. The equator portion of the ball 6 is located on the hypothetical surface which passes through the central point of the ball 6 and is orthogonal to the center axis of rotation. This operation is inconvenient for tiny balls 6 and takes a long time.
It is an objective of the present invention to provide a surface shape evaluation method and device for a spherical body, wherein the spherical body is rotated on a predetermined central axis, and the measurement of shape is conducted at a surface portion of the spherical body located on the hypothetical surface which passes through the central point of the ball 6 and is orthogonal to the center axis of rotation, for judgement, good or not, of the spherical body.
It is another objective of the present invention to provide a surface shape evaluation method and device for a spherical body, wherein the surface shape affecting the performance of the vibration and sound of the ball bearing is highly precisely measured without being affected by the rotation precision of the rotation drive portion of it for reliable evaluation and easy measurement operation.
It is another objective of the present invention to provide a surface shape evaluation method and device for a spherical body, whereby an inexpensive ball bearing with high performance in vibration and sound can be realized for efficient development of such a ball bearing.